A radio transceiver has an ability to both receive and transmit information modulated on a carrier frequency. In general, to provide satisfactory performance, the carrier frequency must be predetermined. In many radios, the carrier frequency used for transmission may be identical to, or may otherwise be a function of, the carrier frequency used for reception.
During reception, a radio can ordinarily compensate for various internal frequency errors (caused, for instance, by component tolerances, age, or temperature) by use of various prior art frequency locking techniques. In general, a receiving radio need only start in the general vicinity of the carrier frequency it seeks to receive, and a locking mechanism, such as a phase locked loop, can be used to precisely locate the reception frequency.
During transmission, however, different circumstances prevail. The carrier frequency transmitted by the radio must be as precise as possible. In a base station radio, this can be accomplished in many ways, including the use of high tolerance (and expensive) components, redundant (and expensive) calibrating and monitoring circuits, and the like. In a mobile radio transceiver, however, where space and cost parameters are very important to the end user, such remedies are often not available. Therefore, the problems of maintaining a correct transmission frequency are greater.
Most of the frequency spectrum dedicated by the Federal Communications Commission to land mobile usage has channel spacing of 25 kHz. Generally available components can be, and are, used in today's mobile radios with reasonable success as regards accuracy of the transmission frequency. Though typically not as accurate as a base station transmission, the width of the channel spacing provided enough latitude to ensure reasonable performance.
In some of the spectrum dedicated by the FCC to land mobile usage, however, the FCC has mandated that channel spacing must be 12.5 kHz instead of the traditional 25 kHz. With such narrow channel spacing, the importance of maintaining a correct transmission frequency has increased.
There therefore exists a need for a means of providing accurate transmission frequencies in a mobile radio. The desired mechanism should not contribute unduly to the expense of the radio nor its complexity of operation. Further, the desired mechanism should not greatly impair the manufacturing process or serviceability of the radio.